Type 1 diabetes (T1D) is one of the most common chronic diseases of childhood throughout the world. It is a complex, multigenic autoimmune disease that is caused by the immune mediated destruction of insulin producing ? cells in the pancreas. Current treatment is exogenous insulin and dietary restriction and, although improving, patients continue to face significant morbidity and decreased life expectancy. Despite being an active area of research, the cause remains unknown and there is no cure. The development of prevention strategies, treatments and a durable cure are essential. An incomplete understanding of the biological mechanisms that lead to and drive T1D pathogenesis has hindered progress. The etiology of T1D is strongly linked to genetic variations within major histocompatibility complex (MHC) class II (MHCII) locus, especially the human leukocyte antigen (HLA)-DR and HLA-DQ genes. Other genes in the class II locus, however, also contribute to the disease. Our collaborators in Sardinia, Italy undertook extensive, new T1D genetic association studies in an attempt to uncover these other players. Due to a founder effect quirk, Sardinians suffer from the second highest rate of T1D in the world, making this genetic pool well suited for these studies. These data have identified an allele of HLA-DM (DM), which controls the loading and editing of the MHCII-bound peptide cargo. Remarkably, this allele of DM is associated with protection from T1D. MHCII presentation of self- peptides is essential for CD4 T cell activation and the initiation of T1D. The repertoire of self-antigens that are displayed by MHCII is dependent on the function of DM. Thus, DM activity clearly has the potential to be central to the development of T1D. The protective allele of DM has a bulky amino acid substitution that localizes to the interface between MHCII and DM, suggesting suboptimal MHCII-DM interactions. This leads us to hypothesize that protection from T1D in individuals expressing this DM allele is due to reduced activity, resulting in the presentation of an altered repertoire of MHCII-bound peptides. This idea is strongly supported by our published studies, which showed that dampening DM activity in Non-Obese Diabetic (NOD) mice is protective against T1D. The goal of the studies proposed in this exploratory R21 application is to test the first part of this hypothesis; the protective DM allele has reduced functional activity when compared to the non- protective DM allele. These ideas will be tested in two specific aims. Aim 1 will use cellular, biochemical and in vitro assays to measure the DM activity of the protective versus the non-protective DM allele. Aim 2 will determine the functional ramifications of the protective DM allele on MHCII-peptide presentation to CD4 T cells. The data from these experiments represent an essential step towards understanding the role of DM in preventing the development of T1D in humans. Successful completion of these studies will identify the MHCII antigen processing pathway and specifically, DM, as a target for intervention and prevention of this chronic disease.